1. Field of the Invention
The present invention pertains to a method and apparatus for processing a video signal, and more particularly to removing (defeating) effects of phase modulation of the color burst component of the video signal.
2. Description of the Prior Art
U.S. Pat. No. 4,577,216, "Method and Apparatus For Processing a Video Signal," John O. Ryan, issued Mar. 18, 1986 and incorporated by reference, discloses modifying a color video signal to inhibit the making of acceptable video recordings thereof. A conventional television receiver produces a normal color picture from the modified signal. However, the resultant color picture from a subsequent video tape recording shows variations in the color fidelity that appear as bands or stripes of color error. Colloquially the modifications are called the "color stripe system" or the "color stripe process". Commercial embodiments of the teachings of this patent typically limit the number of video lines per field having the induced color error or color stripes.
Color video signals (both in the NTSC and PAL TV systems) include what is called a color burst. The color stripe system modifies the color burst. The suppression of the color subcarrier signal at the TV transmitter requires that the color TV receiver include (in NTSC) a 3.58 MHz oscillator which is used during demodulation to reinsert the color subcarrier signal and restore the color signal to its original form. Both the frequency and phase of this reinserted subcarrier signal are critical for color reproduction. Therefore, it is necessary to synchronize the color TV receiver's local 3.58 MHz oscillator so that its frequency and phase are in step with the subcarrier signal at the transmitter.
This synchronization is accomplished by transmitting a small sample of the transmitter's 3.58 MHz subcarrier signal during the back porch interval of the horizontal blanking pulse. FIG. 1A shows one horizontal blanking interval for color TV. The horizontal sync pulse, the front porch and blanking interval duration are essentially the same as that for black and white TV. However, during color TV transmission (both broadcast and cable) 8 to 10 cycles of the 3.58 MHz subcarrier that is to be used as the color sync signal are superimposed on the back porch. This color sync signal is referred to as the "color burst" or "burst". The color burst peak-to-peak amplitude (40 IRE for NTSC TV as shown) is the same amplitude as the horizontal sync pulse.
FIG. 1B shows an expanded view of a part of the waveform of FIG. 1A including the actual color burst cycles. During the color TV blanking intervals, such a color burst is transmitted following each horizontal sync pulse.
In one commercial embodiment of the color stripe process, no color burst phase (stripe) modification appears in the video lines that have a color burst signal during the vertical blanking interval. These are lines 10 to 21 in an NTSC signal and corresponding lines in a PAL signal. The color stripe modifications occur in bands of four to five video lines of the viewable TV field followed by bands of eight to ten video lines without the color stripe modulation. The location of the bands is fixed ("stationary") field-to-field. This color stripe process has been found to be quite effective for cable television, especially when combined with the teachings of U.S. Pat. No. 4,631,603 also invented by John O. Ryan and incorporated herein by reference.
In NTSC TV, the start of color burst is defined by the zero-crossing (positive or negative slope) that precedes the first half cycle of subcarrier (color burst) that is 50% or greater of the color burst amplitude. It is to be understood that the color stripe process shifts the phase of the color burst cycles relative to their nominal (correct) position which is shown in FIG. 1B. The phase shifted color burst is shown in FIG. 1C. The amount of phase shift shown in FIG. 1C is 180.degree. (the maximum possible).
Further, the amount of phase shift in the color stripe process can vary from e.g. 20.degree. to 180.degree.; the more phase shift, the greater the visual effect in terms of color shift. In a color stripe process for PAL TV, a somewhat greater phase shift (e.g. 40.degree. to 180.degree.) is used to be effective.
Other variations of the color stripe process are also possible.
U.S. Pat. No. 4,626,890, "Method and Apparatus For Removing Phase Modulation From the Color Burst", John O. Ryan, issued Dec. 2, 1986 and incorporated by reference, discloses removing the phase modulation of the U.S. Pat. No. 4,577,216. This removal is useful in eliminating much of the effects of the process disclosed in U.S. Pat. No. 4,577,216 for recording.